Monoethylene glycol is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids.
Monoethylene glycol is typically prepared from ethylene oxide, which is in turn prepared from ethylene. Ethylene and oxygen are passed over a silver oxide catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C., producing a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. The amount of ethylene oxide in the product stream is usually between about 0.5 and 10 weight percent. The product stream is supplied to an ethylene oxide absorber and the ethylene oxide is absorbed by a re-circulating solvent stream containing mostly water. The ethylene oxide-depleted stream is partially or entirely supplied to a carbon dioxide absorption column wherein the carbon dioxide is at least partially absorbed by a re-circulating absorbent stream. Gases that are not absorbed by the re-circulating absorbent stream are recombined with any gases bypassing the carbon dioxide absorption column and are recycled to the ethylene oxide reactor.
The solvent stream leaving the ethylene oxide absorber is referred to as fat absorbent. The fat absorbent is supplied to an ethylene oxide stripper, wherein ethylene oxide is removed from the fat absorbent as a vapour stream. The ethylene oxide-depleted solvent stream is referred to as lean absorbent and is recirculated to the ethylene oxide absorber to absorb further ethylene oxide.
The ethylene oxide obtained from the ethylene oxide stripper can be purified for storage and sale or can be further reacted to provide ethylene glycol. In one well-known process, ethylene oxide is reacted with a large excess of water in a non-catalytic process. This reaction typically produces a glycol product stream consisting of almost 90 weight percent monoethylene glycol, the remainder being predominantly diethylene glycol, some triethylene glycol and a small amount of higher homologues. In another well-known process, ethylene oxide is catalytically reacted with carbon dioxide to produce ethylene carbonate. The ethylene carbonate is subsequently hydrolysed to provide ethylene glycol. Reaction via ethylene carbonate significantly improves the selectivity of ethylene oxide conversion to monoethylene glycol.
Efforts have been made to simplify the process for obtaining ethylene glycol from ethylene, reducing the equipment that is required and reducing the energy consumption. GB 2107712 describes a process for preparing monoethylene glycol wherein the gases from the ethylene oxide reactor are supplied directly to a reactor wherein ethylene oxide is converted to ethylene carbonate or to a mixture of ethylene glycol and ethylene carbonate.
EP 776890 describes a process wherein the gases from the ethylene oxide reactor are supplied to an absorber wherein the absorbing solution mainly contains ethylene carbonate and ethylene glycol. The ethylene oxide in the absorbing solution is supplied to a carboxylation reactor and allowed to react with carbon dioxide in the presence of a carboxylation catalyst. The ethylene carbonate in the absorbing solution is subsequently supplied, with the addition of water, to a hydrolysis reactor and subjected to hydrolysis in the presence of a hydrolysis catalyst.
EP 2178815 describes a reactive absorption process for preparing monoethylene glycol wherein the gases from the ethylene oxide reactor are supplied to an absorber and the ethylene oxide is contacted with lean absorbent comprising at least 20 wt % water in the presence of one or more catalysts that promote carboxylation and hydrolysis and the majority of the ethylene oxide is converted to ethylene carbonate or ethylene glycol in the absorber.
Towers or columns allowing the intimate gas-liquid contacting required for such absorption are well known in the art and are referred to, for example, as fractionation, distillation or absorption towers. Such towers or columns contain trays stacked vertically through the column and are designed to conduct liquids in a zig-zag course downwardly through the column while admitting gases upwardly into horizontal-flowing portions of the liquid for intimate contact with the liquid.
Trays for providing the horizontal flow of the liquid are well known in the art and have been widely used. A tray generally comprises a perforated gas-liquid contacting member or members for effecting intimate contact between a gas rising through the tray and a liquid flowing across the surface of the tray across the perforated member. The perforated gas-liquid contacting member is in some instances provided with bubble caps or valves. At one edge of the contacting member of the tray is a liquid inlet area for receiving the liquid onto the tray. This area will generally contain no perforations. At the opposite edge of the contacting member is the liquid discharge end or region of the tray, which is provided with an outlet weir member extending vertically above the surface of the tray. The flowing liquid overflows the outlet weir for discharge from the tray. Accordingly, this outlet weir, maintains a given liquid depth on the tray.
Extending below the trays is one or more downcomer element which, in cooperation with the inner surface of wall of column or tower, forms a downcomer for the passage of liquid downwardly from the tray liquid discharge region or end to the liquid inlet region of the vertically adjacent tray directly below. The downwardly flowing liquid received on the liquid inlet region or area then flows across the surface of this tray in a path across the perforated gas-liquid contacting member, to the tray discharge end or region and is discharged from the tray, over the outlet weir into the next downcomer.
A gas flows upwardly in the column through the perforations of the gas-liquid contacting members of the trays, allowing intimate contact with the liquid flowing horizontally across the surface of the tray. The gas is prevented from passing up the downcomers, as the downcomer element also functions as a baffle extending below the surface level of the flowing liquid to seal the downcomers from gas bypass. However, gas bypassing through downcomers may occur during start up of the process, when the column is not yet sufficiently filled with liquid.
The structure of an individual column and the trays therein must be determined on the basis of the process for which they are intended to be used. For example, outlet weirs in the art vary in height depending on the nature of the operation of the column or tower. U.S. Pat. No. 4,435,595 describes a reactive distillation process for the production of high purity methyl acetate in which high weirs are used. The outlet weirs in this case are 5 inches (12.7 cm) in height.
US 2013/0245318 teaches a rectification column for the production of a methionine salt in which the weirs have a height of 100 mm or more.
EP 1964829 describes a multi-stage distillation column comprising vertically stacked trays having a weir height in the range of from 3 to 20 cm.
The present inventors have sought to provide an improved process for the manufacture of alkylene glycol from an alkene. In particular, the present inventors have sought to provide a process and an absorption system that allows reactive absorption of the gas composition from an alkylene oxide reactor with high selectivity.